1. Field of the Invention
The present invention relates to an apparatus and method for improving the focus control of an optical storage system.
2. Background
Optical storage systems, which include CD drives and DVD drives and other known optical drives, have become very popular with the advent of the numerous multimedia applications that have been introduced into the market. FIG. 1 illustrates the general components of an optical disk drive 10 (such as a CD drive). A laser diode 12 emits a beam of laser that is reflected off a reflector 14 and directed through a lens 16 to impinge on the surface 18 of a disk 20 that contains data to be read. The light is reflected off the surface 18 back through the lens 16 to a sensor 22 which detects the light.
Depending on the nature of the optical path of the light in FIG. 1, the reflected light that is detected by the sensor 22 will either be a perfect dot (see FIG. 2), or an ellipse (see FIGS. 3 and 4). Referring first to FIG. 2, if the reflected light travels through perfect conditions in the optical path, the detected light will be a round dot having a total area that is equally divided (i.e., have the same area) into each of the four quadrants A, B, C and D. In other words, FIG. 2 shows a spot with the areas A+C=B+D. On the other hand, if the lens 16 is positioned at a distance that is closer to the surface 18 than optimum, then the detected light at the sensor 22 will take the form of the ellipse shown in FIG. 3, where A+C<B+D. Similarly, if the lens 16 is positioned at a distance that is further from the surface 18 than optimum, then the detected light at the sensor 22 will take the form of the ellipse shown in FIG. 4, where A+C>B+D. Thus, in FIGS. 3 and 4, the actual spot shifts or deforms from the central point between the four quadrants A, B, C, D.
A number of factors may contribute to “imperfect” conditions that may result in the detected light taking the form of one of the ellipses shown in FIG. 3 or 4. For example, the surface 18 of the disk 20 may be thermally deformed, or the disk 20 may be placed incorrectly to create an angular inclination with respect to the sensor 22. As a result, these changes in the distance between the lens 16 and the surface 18 of the disk 20 need to be compensated for during the reading of the data on the disk 20, so as to ensure accurate data reads, and to minimize the spot size.
To achieve this objective, it is desirable to maintain a minimum spot size on the surface 18, because the smallest spot size of the light on the surface 18 ensures that data is being accurately read, and that surrounding (i.e., not relevant) data is not being added or introduced to the desired data that is being read. Here, the desired data that is to be read would be represented by the “spot”, so that a minimum spot size would provide more accurate data transfer. In the system of FIG. 1, the lens 16 helps to focus the light on to the smallest spot on the surface 18.
FIG. 5 is a graphical illustration of an S-curve of a focus error (FE) signal that can be used for focusing control. This S-curve is an industry standard that is used by most manufacturers to construct the focus control of their optical storage systems. The S-curve in FIG. 5 represents the relative physical distance x between the lens 16 and the reflection surface 18. In this regard, the center of the FE signal shown in FIG. 5 has a value of FE=0, which represents the electrical or originally assumed ideal condition shown in FIG. 2. One can set the regulation point to be FE=0, although any point along the S-curve can be designated to be the regulation point. Since FE is a voltage, the manufacturer can turn on the focus servo and watch the RF signal's AC amplitude vary as the power of the reflection signal changes. Thus, referring to FIG. 5, the FE value can be offset positively (i.e., to the right of the S-curve) or offset negatively (i.e., to the left of the S-curve) to change the distance x. Under the well-known defocus procedure, one can change the focus control from FE=0 to any physical position represented along the S-curve to determine the RF signal power generated at various points along the S-curve so as to determine the point MAX that yields the maximum signal energy that is received by the sensor 22. This is because the minimum spot size can be achieved by maximizing the signal energy of the reflected light that is received by the sensor 22. Under the defocus procedure, the difference between the values of the distance x (from FE to FE′ as shown in FIG. 5) represents a delta FE value that can be used to find the maximum signal energy.
Unfortunately, there are applications (e.g., CD-R or CD-RW recording procedures) where the defocus procedure cannot be used. For example, the deformation of the reflection surface cannot be conveniently measured in advance, and the deformation sometimes occurs during recording. Thus, there still remains a need for an apparatus and method for improving the focus control of an optical storage system, which can be applied to any application.